This invention relates to X-ray tubes having rotating anodes and more particularly to a retainer for supporting the bearings used in X-ray tubes.
A principal component of conventional X-ray equipment and computed tomography (CT) equipment is an X-ray tube which provides the source of X-rays. Such tubes contain a vacuum at 10.sup.-8 to 10.sup.-9 torr and operate by accelerating a stream of electrons from a heated cathode through a high voltage against a target anode. The conversion efficiencies of such tubes are low and therefore considerable heat is generated in the anode as a by-product of the X-rays generation.
In order to reduce heat concentration in the anode, the anode is rotated at speeds up to 10,000 RPM thereby continuously presenting the cathode a new and cooler surface. In a high performance X-ray tube, the surface of the anode may reach temperatures of 3200.degree. C., and areas of the anode outside the immediate target surface may rise to temperatures of approximately 1300.degree. C.
Much of the heat generated in the anode is radiated through the glass walls of the tube from high emissivity anode coatings Even so the anode shaft and support bearings on which the anode rotates, may rise to temperatures of up to 450.degree. C. It should be noted that the anode shaft and bearings are contained within the evacuated X-ray tube envelope.
To prolong bearing life it is generally known to preload the bearings supporting the anode shaft. Typically, a front bearing is held fixed with respect to the anode stem and the rear bearing's outer race is held in a retainer that is free to slide axially within a hollow anode stem. The rear bearing's inner race 46 is affixed to the anode shaft. A preload spring applies an axial force to the rear retainer to provide preloading to both bearings. The preloading force improves the tracking of the bearing ball sandwiched between the inner and outer races of both front and rear bearings, increasing bearing life and reducing bearing noise.
To prevent excessive radial play in the anode shaft, the sliding bearing retainer holding the rear bearing must fit closely within the guiding anode stem. The close fit between the bearing retainer and the anode stem raises a number of problems including that of friction between the sliding surface of the retainer and the anode stem altering the bearing preload, wear between the sliding surfaces which may increase the radial play in the anode shaft, and the generation of "wear" particles which may contaminate the bearing causing increased bearing noise and wear. Bearing noise is thought to arise in part from particulate matter either from foreign substances incorporated into the bearing during the bearing manufacture or particles shed from the bearings themselves or other sliding tube components.
The combined effect of high rotational speed, high operating temperatures and a vacuum environment places severe demands on the anode support bearings. Accordingly, bearing failure is the chief limit to X-ray tube life. Such failure may result either from bearing "freeze", a seizure of the rolling elements of the bearing within their races, or from material transfer from the anode stem to the bearing retainer hindering the sliding motion between the stem and retainer surfaces, or from an increase in bearing noise which may necessitate tube replacement despite otherwise acceptable operation.
The extreme operation temperatures and high vacuum environment to which the retainer and anode stem are subjected prevents the use of conventional organic lubricants in this application.